"This isn't a simple, easy matter of let's just move 122 detainees," he said.
Conservative senator says Obama should give the independent "full and fair consideration."
The Defense secretary said the group's gains had been "reversed" in some areas.
One of the bases reportedly is within 100 miles of the Syrian border.
The House will vote Wednesday on the measure and a stopgap spending bill.
The Pentagon said Hagel had asked for more information on the program.
Army Gen. John Campbell’s has been tapped to command U.S. and NATO forces.
The plan won't include money for Iraq unless Obama approves military action.
The White House shot down the idea of bringing Khattala to Gitmo.
Lawmakers voted 93-3 in favor of the legislation.
The bill includes language prohibiting the use of funds to transfer prisoners from Gitmo.
The administration on Tuesday came under withering criticism over the swap.
Insiders say Hagel's departure was the culmination of a larger power struggle.
Lawmakers are wading deeper into the defense issues they left on the backburner.
"Defense leaders must be part of this global discussion," said Hagel.
Obama thanked lawmakers “for the speed and seriousness" they brought to the issue.
GOP senators demanded the administration produce a plan to deal with U.S. passport holders who have...
Although the president has authorized the deployment of about 905 U.S. troops to Iraq, and the...
The expensive fleet remained grounded as the Pentagon investigated an onboard fire.
Hagel and Dempsey express bitter disappointment with Iraqi government.
The combat ship is home to 500 Marines and a number of attack helicopters.
The White House sought to deflect suggestions that Hagel made decision.
Nine senators and 25 House members will make the transatlantic trip.
Republicans want to know if Obama and Clinton talked about the prisoner swap.